Six-stroke engine cycles are known in the art, and consist generally of an intake stroke, a first compression stroke, a first power stroke, a second compression stroke, a second power stroke, and an exhaust stroke. During the intake stroke, air or a mixture of fuel and air is drawn into an engine's combustion chamber. During the first compression stroke, the mixture is compressed and, toward an end of the first compression stroke, ignited. During the first power stroke, the burning mixture causes the volume of the combustion chamber to expand rapidly by pushing an associated piston downward, thereby creating useful mechanical work output. During the second compression stroke, residual combustion gases and/or new fuel and air are again pressurized and ignited. During the second power stroke, the burning mixture again expands the volume of the combustion chamber to create useful mechanical work output. During the exhaust stroke, byproducts of the combustion processes are pushed from the combustion chamber by the piston.
Historically, both of the combustion events described above (i.e., the combustion events occurring during the first and second power strokes) have been similar during a single cycle. That is, the air/fuel mixture combusting during both events has had substantially the same ratio of air-to-fuel. To provide this same ratio, engine controls such as valve timing and fuel injection are maintained generally consistent during the different combustion events by way of mechanical cam mechanisms. Although effective, this operation may be less than optimal with regard to efficiency, power, and exhaust emissions.
One attempt to improve operation of a six-stroke engine is disclosed in US Patent Publication No. 2007/0044778 (the '778 publication) by Milovanovic et al. published on 1 Mar. 2007. The '778 publication describes a direct injected engine having a first stage of combustion and a second stage of combustion. During the first stage of combustion, a rich ratio of fuel and air are supplied to a combustion chamber of the engine and combusted by a compression ignition diesel process. A majority of the combusted gases resulting from the first stage of combustion are retained in the combustion chamber, and additional air is supplied to the combustion chamber. The resulting mixture having a lean ratio is then combusted during a second combustion stage by homogenous charge compression ignition. The first stage of combustion, because of its rich nature may provide increased power output, while the second stage, because of its lean nature, may reduce exhaust emissions.
Although the '778 publication may provide for enhanced performance of an engine during six-stroke operation, it may still be less than optimal. Specifically, the rich ratio of fuel and air during the first stage of combustion may produce exhaust emissions in an amount too great to be sufficiently reduced during the second stage of combustion.
The engine control system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.